The number of people who acquire HIV through heterosexual sex in the UK is set to keep rising, experts warn.

Health Protection Agency data published in the British Medical Journal has shown the number rose from 144 in 1999 to 315 in 2003.

The vast majority of heterosexual men and women diagnosed with HIV in the UK are infected abroad.

But as the numbers living with HIV increased, so would heterosexual transmission risks, observers said.

Homosexual men remain at the greatest risk of acquiring HIV within the UK.

We cannot afford to be complacent

Terrence Higgins Trust spokeswoman

The HPA Centre for Infections analysed confidential reports of HIV infection in England, Wales and Northern Ireland to look at probable routes of infection.

Of the 21,115 adults diagnosed with HIV between 1985 and 2003, 9% were likely to have been infected within the UK.

The rest includes people who are UK born who were infected when they travelled abroad, or people from countries with high HIV rates, mainly in Africa, who were diagnosed with HIV after moving to the UK.

'Dispelling the myths'

Dr Katy Sinka, a senior epidemiologist from the HPA, told the BBC News website: "Our main concern is that infections acquired in the UK continue to increase very gradually.

"We should be able to target prevention strategies in the UK, when we can't necessarily act to prevent infections occurring abroad."

The researchers fear that, as the number of heterosexuals living with HIV in the UK grows, "the likelihood of heterosexual transmission within the country will increase, particularly among ethnic minorities."

A spokeswoman for the Terrence Higgins Trust said: "This research confirms that although gay men are at the greatest risk of HIV infection, the risks for the heterosexual community are steadily growing, especially for the African community living in the UK.

"We cannot afford to be complacent. Both health education and wider availability of HIV testing and treatment are essential if we are to tackle the UK's growing epidemic."

Michael Carter, of the National Aids Manual, said: "These figures go some way to dispelling some myths that are developing about the epidemiology of HIV in the UK.

"It's important to emphasise that they show that gay men still account for the overwhelming majority of infections acquired in the UK. However, heterosexual transmission within the UK is increasing."

He said a major factor in the continuing rise was that a third of all of people with HIV in this country do not know they are infected. He advised everyone in the UK who thought they might be at risk to be tested.

"It's important to add that even if tests indicate that HIV treatment had reduced HIV to undetectable levels, they shouldn't assume that this means that they are no longer infectious.

"When properly used, condoms are an excellent way of preventing the transmission of HIV and other sexually transmitted infections.